132 research outputs found
S-matrix studies of resonances in A=3,4,5,6, and 12 nucleon systems
Resonances of certain light nuclei are explored by studying the complex pole
structures of the scattering matrices. Among other results we predict the
existence of three-neutron and three-proton resonances, a small spin-orbit
splitting in the low-lying He-5 and Li-5 states and the nonexistence of the
soft dipole resonance in He-6.Comment: Talk given at the XVth International Conference on Few-Body Problems
in Physics (22-26 July, 1997, Groningen, The Netherlands). To be published in
the conference proceedings in Nucl. Phys. A. 5 pages with 2 figures. The
postscript file and more information are available at
http://qmc.lanl.gov/~csot
Elastic p-3He and n-3H scattering with two- and three-body forces
We report on a microscopic calculation of n-3H and p-3He scattering employing
the Argonne v_{18} and v_8' nucleon-nucleon potentials with and without
additional three-nucleon force. An R-matrix analysis of the p-3He and n-3H
scattering data is presented. Comparisons are made for the phase shifts and a
selection of measurements in both scattering systems. Differences between our
calculation and the R-matrix results or the experimental data can be attributed
to only two partial waves (3P0 and 3P2). We find the effect of the Urbana IX
and the Texas-Los Alamos three-nucleon forces on the phase shifts to be
negligible.Comment: submitted to Phys. Rev.
Ground--state energies and widths of He and Li
We extract energies and widths of the ground states of He and Li from
recent single--level R--matrix fits to the spectra of the H)He and the He)Li reactions. The widths
obtained differ significantly from the formal R--matrix values but they are
close to those measured as full widths at half maxima of the spectra in various
experiments. The energies are somewhat lower than those given by usual
estimates of the peak positions. The extracted values are close to the
S--matrix poles calculated previously from the multi--term analyses of the
N-He elastic scattering data.Comment: 3 pages, no figures, uses revtex.sty, accepted for publication in
PRC, uuencoded postscript and tex-files available at
ftp://is1.kph.tuwien.ac.at/pub/ohu/fwidth.u
On Pole Assignment and Stabilizability of Neutral Type Systems
In this note we present a systematic approach to the stabilizability problem of linear infinite-dimensional dynamical systems whose infinitesimal generator has an infinite number of instable eigenvalues. We are interested in strong non-exponential stabilizability by a linear feed-back control. The study is based on our recent results on the Riesz basis property and a careful selection of the control laws which preserve this property. The investigation may be applied to wave equations and neutral type delay equations
Microscopic Calculation of the System
We report on a consistent, microscopic calculation of the bound and
scattering states in the 4He system employing a realistic nucleon-nucleon
potential in the framework of the resonating group model (RGM). We present for
comparison with these microscopic RGM calculations the results from a
charge-independent, Coulomb-corrected R-matrix analysis of all types of data
for reactions in the A=4 system. Comparisons are made between the phase shifts,
and with a selection of measurements from each reaction, as well as between the
resonance spectra obtained from both calculations. In general, the comparisons
are favorable, but distinct differences are observed between the RGM
calculations and some of the polarisation data. The partial-wave decomposition
of the experimental data produced by the R-matrix analysis shows that these
differences can be attributed to just a few S-matrix elements, for which
inadequate tensor-force strength in the N-N interaction used appears to be
responsible.Comment: 53 pages, PS document can also be found at
ftp://theorie3.physik.uni-erlangen.de/pub/publications/he4.ps.g
Coupled-channel effective field theory and proton-Li scattering
We apply the renormalisation group (RG) to analyse scattering by short-range
forces in systems with coupled channels. For two S-wave channels, we find three
fixed points, corresponding to systems with zero, one or two bound or virtual
states at threshold. We use the RG to determine the power countings for the
resulting effective field theories. In the case of a single low-energy state,
the resulting theory takes the form of an effective-range expansion in the
strongly interacting channel. We also extend the analysis to include the
effects of the Coulomb interaction between charged particles. The approach is
then applied to the coupled Li and Be channels which couple to
a state of Be very close to the Be threshold. At
next-to-leading order, we are able to get a good description of the Li
phase shift and the Be(n,p)Li cross section using four parameters.
Fits at one order higher are similarly good but the available data are not
sufficient to determine all five parameters uniquely.Comment: 22 pages, 2 figures, RevTeX4, typos corrected, accepted for
publication in European Physical Journal
Perceptions of 'coming out' among British Muslim gay men
The cultural processes of heteronormativity and compulsory heterosexuality are acutely active within Islamic societies. The present study explored perceptions of 'coming out' among a group of British Muslim gay men (BMGM), focussing upon the potential consequences for identity processes and psychological well-being. Ten BMGM of Pakistani descent were interviewed using a semi-structured interview schedule. Interview transcripts were subjected to interpretative phenomenological analysis and informed by identity process theory. Four superordinate themes are reported, including (1) 'social representational constraints upon "coming out"'; (2) ' "coming out": a source of shame and a threat to distinctiveness'; (3) 'fear of physical violence from ingroup members'; and (4) 'foreseeing the future: "coming out" as a coping strategy'. Data suggest that BMGM face a bi-dimensional homophobia from ethno-religious ingroup members and the general population, which can render the prospect of 'coming out' threatening for identity. Theoretical and practical implications of this research are discussed
Neutron cross sections for carbon and oxygen from new R-matrix analyses of the 13,14
We report the latest results from R-matrix analyses of reactions in the 13,14C and 17O systems that are of interest in reactor applications and nuclear astrophysics. These were done in order to provide separate cross sections for the stable isotopes (12,13C) of natural carbon, and to contribute improved cross sections for 16O to the CIELO project. Although particular attention was paid to the data in the standards region (<2 MeV) for the carbon isotopes, and to the low-energy region for n+16O, the analyses extend to several MeV neutron energy for all the systems. The fits to the data included are generally quite good, in keeping with the unitary constraints of R-matrix theory. The cross sections for 12,13C give results for natural carbon that are close to the previous evaluation by Fu et al. at energies below 1 MeV. Above that energy, the deviations become larger, especially near the narrow resonances. The thermal cross section for 16O is at the upper end of the range of recommended values, in excellent agreement with a high-precision measurement by Schneider. At higher energies, the 17O analysis follows in great detail high-resolution measurements of the total cross section, and agrees quite well with the 13C(α,n)16O cross section measurement of Bair and Haas at roughly their original normalization scale. We will discuss the implications of these new evaluations for critical benchmarks and astrophysical applications
An Examination of Chimpanzee Use in Human Cancer Research
Advocates of chimpanzee research claim the genetic similarity of humans and chimpanzees make them an indispensable research tool to combat human diseases. Given that cancer is a leading cause of human death worldwide, one might expect that if chimpanzees were needed for, or were productive in, cancer research, then they would have been widely used. This comprehensive literature analysis reveals that chimpanzees have scarcely been used in any form of cancer research, and that chimpanzee tumours are extremely rare and biologically different from human cancers. Often, chimpanzee citations described peripheral use of chimpanzee cells and genetic material in predominantly human genomic studies. Papers describing potential new cancer therapies noted significant concerns regarding the chimpanzee model. Other studies described interventions that have not been pursued clinically. Finally, available evidence indicates that chimpanzees are not essential in the development of therapeutic monoclonal antibodies. It would therefore be unscientific to claim that chimpanzees are vital to cancer research. On the contrary, it is reasonable to conclude that cancer research would not suffer, if the use of chimpanzees for this purpose were prohibited in the US. Genetic differences between humans and chimpanzees, make them an unsuitable model for cancer, as well as other human diseases
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